A coating may be defined as a layer of a substance which is used to protect and/or decorate a surface. It must adhere to a substrate, in contrast to a covering, which may simply surround or lie over the surface of such a substrate without bonding thereto. Coatings are primarily available as liquid or fusible compositions wherein the liquid portions are usually aqueous or organic solutions. Coating are meant to be applied to the surface of a substrate to be treated. The liquid portion enables the coating to flow smoothly over the surface of the substrate, chiefly due to forces of surface tension, before evaporating or reacting with the other coating components to form a solid, tightly adherent composition thereon.
At the time of application, coatings generally include an organic polymeric binder, one or more pigments, a volatile thinner or solvent and various additional additives. Each ingredient, in itself, may be a complicated mixture, depending upon the end use of the coating.
The binder is ordinarily a film-forming organic polymer, such as cellulose nitrate, an alkyd resin, or a mixture of such polymers, having glassy, plastic, or rubbery properties in the dried state. Binders are grouped into certain overlapping classes, such as acrylic, vinyl, alkyd, polyester, etc. The structure of the binder molecules and the forces operating between the molecules determines the properties of these binders. The polymeric binder thus adheres to the substrate and also binds the pigment particles, if present, into a coherent film.
The pigments are inorganic or organic powders of varying hiding power and color. They are practically insoluble in the binder and confer color, opacity, and a variety of mechanical and physical properties to the film.
The volatile thinner or solvent, which evaporates after the coating is applied, is used to reduce the consistency of the coating so that it may be brushed, sprayed, calendered, dipped, electrocoated, or otherwise applied to the surface of a substrate. Volatile thinners include a variety of organic liquids, such as esters, alcohols, ketones, naphthas, or paraffin hydrocarbons, depending upon the required solubility characteristics of the particular binder. The thinner may be water, as, for example, in emulsion coatings, or an organic nonsolvent, as in organosols.
Small percentages of other materials may also be included within the above described coating compositions as additives, such as metallic driers for coatings that dry by oxidation, agents to stabilize pigment dispersions and agents to reduce or eliminate foaming of the composition during mixing.
One common type of coating composition is paint, which is made up of pigments blended intimately with a binder. The film-forming materials, i.e., mainly the pigment and binder, are together called the nonvolatiles, or solids. A binder, with or without a thinner, but without pigment, can be generally referred to as a varnish, or sometimes as a "clear lacquer."
Some coatings for specialized applications are applied as monomers and then polymerized in situ. However, most coatings applied as liquids form dry films in other ways. Air-drying coatings harden by evaporation of the thinner on exposure to ventilated atmospheres at room temperature; examples of these are shellac, oxidizing alkyds, and cellulose nitrate lacquers. Other coatings, such as those based on urea-formaldehyde or melamine-formaldehyde resins, undergo condensation polymerization reactions. This usually requires the application of heat.
When temperatures above 150.degree. F. are used to dry a coating, the resultant composition is said to be a baked coating. Some coatings, such as those based on linseed oil, form dry films by way of polymerization reactions which are induced by absorption of atmospheric oxygen, leading to the subsequent gelation and hardening of the composition. Still others may be applied at elevated temperatures without a volatile solvent and dried by cooling them to room temperature. An example of this type is the vinyl copolymer coating often calendered into fabrics.